1. Field of the Invention
The present disclosure relates to a ridge semiconductor laser in which a plurality of semiconductor layers are stacked thereon and a method for manufacturing the ridge semiconductor laser.
2. Discussion of the Background Art
There has been known to use an etching stopper layer when a ridge structure is formed by etching a layer made of an InP compound in a process of manufacturing a ridge semiconductor laser (for example, see JP 2001-144381 A). The etching stopper layer in the JP 2001-144381 A is formed of an AlGaInAs compound and it is exposed to air after formation of a ridge, so that crystal quality could be deteriorated by oxidation of Al.
Thus, an InGaAsP compound has been attempted to be used in the etching stopper layer (for example, see JP 2002-026453 A). Since this etching stopper layer does not contain Al, the deterioration of the crystal quality due to oxidation does not occur even though the etching stopper layer is exposed to air after the ridge formation. In addition, although a carrier stopper layer in the JP 2002-026453 A is an AlInAs compound, the etching stopper layer made of the InGaAsP compound covers directly above the carrier stopper layer not to be exposed in the air in the middle of a process. Consequently, the deterioration of the crystal quality of the carrier stopper layer due to the oxidation of Al does not occur.
In a distributed feedback laser (DFB laser) having an embedded diffraction grating at a ridge portion, when the embedded diffraction grating is formed by a wet etching method, it is necessary to prevent from mixing As in a ridge forming layer made of an InP compound under the diffraction grating in order to improve reproducibility of selective etching. Thus, a growth temperature of the ridge forming layer is required to be lowered. Meanwhile, in an Al-containing semiconductor layer such as an active layer made of an AlGaInAs compound, the growth temperature is required to be increased to a high temperature to enhance the crystal quality.
Thus, after the etching stopper layer between the Al-containing semiconductor layer such as an active layer and the ridge forming layer is stacked, it is necessary to lower the growth temperature and grow the ridge forming layer. Then, since the etching stopper layer contains two V-group elements (As and P), a temperature lowering process of lowering the growth temperature in an AsH3 gas atmosphere and a PH3 gas atmosphere is required for the purpose of protecting a growth surface of the etching stopper layer. However, the decomposition efficiencies of the two gases are different, and thus it is difficult to protect the surface of the etching stopper layer, so that there has been a problem that the crystal quality is easily deteriorated.
Thus, an object of the present disclosure is to provide a ridge semiconductor laser, which has a structure that can improve the reproducibility of the formation of a diffraction grating during manufacturing and prevent deterioration of a crystal quality of an etching stopper layer, and a method for manufacturing the ridge semiconductor laser.